<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <button>按钮1</button>
    <button>按钮2</button>
    <button>按钮3</button>

    <script>
        let but = document.querySelectorAll("button");
          for(let i=0;i<but.length;i++){
            but[i].onclick = function(){
                alert(i);
            }
          }
    </script>
</body>
</html>